1) A popular word in Yorkshire where it is typically used now of the weather and health. It means ‘average’ or ‘medium’; that is nothing to get excited about.
The OED has examples from 1456, and references in Yorkshire date from the early sixteenth century: 1532 a mydlyng bordclothe and ij herden, Hawton
1551 John Swyndell doith owe me fyve punde of mydlyn wyar, Garforth
1599 xv payre of lynen, mydlen and harden sheetes, Rawmarsh. Later quotes reflect a broadening of its usage: 1750 7 acres … as good a crop of oats… as ever he knew … the other four Acre but Midling, Beeston
1786 Aug. 21 At Thirsk. Middling day, showers, Sessay. The 1750 reference seems to anticipate the use of ‘nobbut middlin’ an almost grudging indication that things are going reasonably well.